1. Field of the Invention
The present invention relates to an ink set having a plurality of colors of ink capable of forming a stable discharge state, with which a substantially uniform amount of each ink can be discharged, and a recorded image with virtually no difference in the size of the dots between colors can be obtained, and to a method for manufacturing this ink set, and to an ink jet recording apparatus.
2. Description of the Related Art
Ink jet recording is a method in which droplets of ink are discharged from a fine nozzle to record text or graphics on the surface of a recording medium. Ink jet recording methods that have seen practical use include a method in which electrostrictive elements are used to convert electrical signals into mechanical signals, the ink stored in a nozzle head is intermittently discharged, and text or symbols are recorded on the surface of a recording medium, and a method in which the portion of the ink stored in the nozzle head that is very close to the discharge area is rapidly heated to generate bubbles, the volumetric expansion of these bubbles intermittently discharges the ink, and text or symbols are recorded on the surface of a recording medium.
Stable discharge is one of the requirements of inks used in ink jet recording such as this. Both dye inks and pigment inks are used for ink jet recording inks, but since dye inks that are used include direct dyes, acid dyes, and basic dyes, special care must be taken with a water-soluble ink jet recording ink because it dissolves uniformly in water and is substantially a Newtonian fluid, so its viscosity is not dependent on shear force. The use of a pigment ink for just black ink has also been studied, and in this case penetration is reduced and the ink may be dried on the paper or other medium, so the pigment concentration is only about 2 to 3 wt %, there is no problem with the ink being substantially a Newtonian fluid, and no special care is required by viscosity. However, when penetration is thus reduced, drying takes longer, which is contrary to the goal of high-speed printing. Also, if the other colors are printed at the same time, color mixing occurs at the boundaries, which lowers print quality.
Therefore, what is particularly problematic is that if the amount in which pigments, polymer fines, and the like are added is increased a certain amount in order to ensure a good penetration rate, the viscosity at the ink discharge site ends up being different from the viscosity at the supply site, which means that something like a special mechanism or electrical circuit is required to achieve stable discharge, and this puts an extremely high load on the apparatus.
Also, with prior art, the paper permeability of ink was low, and with a method for suppressing wetness on the surface of the paper, bleeding occurred with ordinary paper, and especially with the recycled paper that has become so popular in recent years. In addition, because the print took so long to dry, when continuous printing was performed, the ink printed on the paper tended not to dry, which prevented the next printed sheet from being stacked on top of the previous sheet. Another problem was that in multicolor printing the adjacent colors would mix together, causing the letters to bleed.
Also, recycled paper is a blend of many different kinds of paper, the permeabilities of which can vary, and this difference in permeation rate also makes bleeding worse. However, if the paper or other printing medium is heated during printing, it takes time for the heating component of the apparatus to reach the required temperature, the apparatus consumes more electrical power, and such a system also tends to damage paper and other printing media.
Further, when paper coated with an ordinary sizing agent or the like is used as the printing medium and printed with an ink containing pigment, the pigment will remain on the surface of the paper, and scuffing resistance suffers. Also, when the surface tension of an ink was high as in the past, this limited the types of paper on which uniform printing was possible, and this tended to decrease the quality of the printed images. One way to deal with this is to increase the permeation rate and add a larger amount of pigments, etc., but when the permeation rate is raised and the amount in which pigments, polymer fines, and so forth are added is increased by a certain amount, this causes the viscosity at the ink discharge site to be different from the viscosity at the supply site, resulting in unstable discharge and requiring a specific mechanism, electrical circuit, etc., and this puts an extremely high load on the apparatus.